Known in the art are vibration motors comprising a working piezoelectric cell connected with the output of an electric oscillator and a feed-back piezoelectric cell connected with the sync input of the same oscillator. The working and feed-back piezoelectric cells are in contact with each other and one of them, in turn, contacts one of the stages of a concentrator of ultrasonic vibrations, whose driving stage is held pressed to one of the surfaces of the rotor (see, for instance, the USSR Inventor's Certificate No. 526012, IPS G11B 25/00, 1976).
Use is made in the said prior art vibration motors of a rod-like concentrator of ultrasonic vibrations, whose driving stage is held firm by an electromagnet against the side surface of a rotor and the other stage is in close contact with a feed-back piezoelectric cell which, in turn, contacts a working piezoelectric cell.
Due to variation of the piezoelectric cell parameters and concentrator-to-rotor contact zone with time and to instability of the oscillator electric parameters in the vibration motors of the type under consideration, it is difficult to attain a high degree of stability and uniformity of rotation frequency of the rotor in such vibration motors for a long period of time.
To reduce durable instability of the rotor rotation frequency, it is necessary either to exclude the effect of destabilizing factors or to provide automatic control of rotation frequency.
To reduce rotation frequency variations of the rotor, it is expedient to diminish the force pressing the concentrator to the rotor with a resultant greater positive effect of the rotor inertia. Reduction of the hold-down force deteriorates starting characteristics of the vibration motor and brings about an increase in durable instability of rotation frequency due to a greater effect of upsetting factors acting on the rotor. Any reduction of the hold-down force accompanied by a greater inertia effect of the vibration motor prolongs transient operating conditions, thereby narrowing the possibility of functional control of rotation frequency of the rotor.